Fatty Acid Dynamics In The Fish Gulf Menhaden

“Essential fatty acids” are critical to organismal growth and development and, because they cannot be synthesized, they must be obtained through the diet. The role of essential fatty acids have been known to the medical community for decades – these components of the diet play a critical role in human health, growth, and development.

The well known omega-3 fatty acids are one example and are used to reduce the risk of heart disease, pain from rheumatoid arthritis, and depression. Other animals, like fishes, also rely on essential fatty acids for their growth and development, and the importance of fatty acid composition is critical in the success in aquaculture because they play an important role in growth, reproduction, and swimming abilities of fishes.

Omega-3 fatty acids like EPA and DHA have recognized physiological functions and are critical during endogenous and early exogenous feeding. Poor growth and survival may occur without sufficient levels of essential fatty acids. Because these biological precursors are so important to organismal development, we sought to describe the intra-annual changes in the fatty acid composition of Gulf Menhaden. This small fish is termed a “forage fish” because it is thought to be a major prey item, and a major ecological role, to a variety of avian, fish, and mammal predators. The Gulf Menhaden stock also supports the largest commercial fishery in the Gulf of Mexico. This fishery is a called a “reduction” fishery because it converts the live fish to both fish oil and fish meal. Fish oil from Gulf Menhaden is rich in EPA and DHA and is widely used for human consumption. Similarly, the fish meal produced is used in a variety of aquaculture and agriculture applications. The implications of seasonal variation in the characteristics of fish oil and fatty acid composition have implications to the dynamics of the reduction fishery as well as to the predators that rely on Gulf Menhaden. In this work, we describe general and specific characteristics of oil content and fatty acid composition of Gulf Menhaden.

To understand the seasonal dynamics of oil content, we obtained fish from the commercial reduction fishery and from biologists that sample the Gulf Menhaden fishery throughout the Gulf of Mexico. Fresh and frozen tissues were evaluated for oil content and processed for the determination of fatty acid composition. The extraction and analysis of fatty acids involved freeze drying and extraction of oil for identification using gas chromatography.
Our analysis indicated the presence of significant seasonal variation in the temporal dynamics of oil content and fatty acid composition of Gulf Menhaden. The seasonal contrasts we observe are an effect of the interaction of the stock with the variation of phytoplankton productivity in the northern Gulf of Mexico and the seasonal movement of the stock between the shelf and the coastal zone.

In the northern Gulf of Mexico, the composition of surface-level phytoplankton taxa is controlled by variations in salinity, temperature, and light conditions, all of which exhibit intra- and inter-annual variability. The enrichment and depletion of fatty acids among consumers follow seasonal patterns: individuals feeding on diets rich in diatoms and dinoflagellates are enriched during cooler months, and depleted cyanobacteria are more abundant during the warmer summer months. Similarly, the observed contrast in seasonal oil content and monthly fatty acid composition may be a result of the movement dynamics of the Gulf Menhaden stock. Gulf Menhaden exhibit seasonal migration, occupying the coastal zone in the mid-spring to early fall and then moving to the inner- and mid-shelf in the late fall, residing and spawning in offshore waters until the following spring.

At the ecosystem level, the contrast in the energy density of Gulf Menhaden observed between seasons indicates that the value of Gulf Menhaden as a prey item and to the fishery in the northern Gulf of Mexico varies substantially during the year.